1. Field of Endeavor
The present invention relates to lasers, and more particularly to a high power laser with a trivalent titanium liquid host.
2. State of Technology
There are many applications for very high average optical power lasers. These applications include power beaming, laser guide stars, illuminators, material processing, remote sensing, laser weapons, and tactical defense systems. A problem in high power laser applications is the lack of suitable sources of radiation with high beam quality. For weapons applications the Chemical Oxygen Iodine Laser (COIL) seems to show promise, but it is a complex and expensive system for use in the 10–100 kW regime. Also at 1.3 micron, it is not useful for use with solar cells and is not readily frequency converted to short wavelengths as would be required for guide stars. Free electron lasers (FELs) have been touted as a solution for decades but have failed to demonstrate useful output powers and acceptable system efficiencies at the wavelengths of interest. Proposed high efficiency recirculating FELs, in which the beam energy is recovered after exiting the wiggler, have been unimpressive due to the poor emittance of the recovered beam and the resulting energy loss in the accelerator and storage ring. To date FELs have yet to produce multi-kilo watt beams at any wavelength and require an immense physical plant to produce even a few watts in the infrared.
Neodymium and other metal ions embedded in a variety of glassy and crystal hosts have been the mainstay of commercial lasers and fusion lasers for decades. However, when these devices are applied to multi-kilowatt applications results have been disappointing. Despite of the development of many complex and expensive systems, practical operation at much greater than one kilowatt with beams near the diffraction limit has not been demonstrated. This complex problem is essentially an issue of thermal management. Solids must have the heat conducted away and at high heat loads this leads to unavoidable temperature gradients. These gradients destroy optical beam quality and can lead to fracture of the media. To cope with these problems designs have recently been put forward that use reduced crystal thickness to improve heat conduction. This approach has limited excited volume, and a complex optical train must be constructed to give the volume needed to produce high average powers. It is not clear if closure can be reached with these designs, since each added component adds additional optical aberrations and opportunities for catastrophic optical damage.
Fluids, on the other hand, have the advantage that when the media gets hot it can be removed from the optical path to a place where it can be cooled. A liquid laser is shown and described in U.S. Pat. No. 3,717,825 to Carl Zeiss-Stiftung, Wuerttemburg, Federal Republic of Germany, patented Feb. 20, 1973. This patent shows a dyestuff laser provided with a liquid guiding chamber through which circulates a cooled laser liquid. The laser is provided with a U-shaped laser active zone formed by a light transmitting longitudinal cap into which extends a tongue forming in said cap a U-shaped zone of uniform cross section. This U-shaped laser active zone is disposed in a focal line of an elliptically shaped pump light reflector while a source of pump light is disposed in the other focal line.
Fluids do not exhibit birefringence so all polarization options are available, have high optical damage thresholds, and do not permanently damage if the threshold is exceeded. Compared to solid laser media fluids are quite inexpensive. U.S. Pat. No. 3,931,594 to Fritz Peter Schafer, assigned to Max-Planck-Gesellschaft zur Forderung der Wissenschaften e.V., patented Jan. 6, 1976, shows a transverse-flow cell for a laser. The cell of a liquid laser is defined between the rounded, parallel edges four coaxial cylinder sectors of fused quartz. The narrow gaps circumferentially bounded by the flat, converging side faces of the sectors provide conduits for rapid flow of dye liquid into and out of the cell which extend over the full axial length of the cell. The light of a flash tube is directed toward the cell through the cylindrically arcuate outer face of each sector.
U.S. Pat. No. 3,740,665 to Irving Itzkan, assigned to Avco Corporation, patented Jun. 19, 1973, shows a transverse flowing liquid laser. Stimulated emission of radiation (laser action) is produced in materials generally classed as dyes. These dyes are dissolved in a liquid solution. A quantity of dye in a flowing liquid solution in a module or cavity is pumped or excited by a laser beam radiating in the ultraviolet region which is focused to a line with a cylindrical lens. A rectangular cross sectional beam of such radiation is produced by a pulsed crossed field nitrogen gas laser. The focused line which is transverse to the beam produced by the exciting laser, and transverse to the direction of the flowing dye, lies near the surface of the dye material in the cell and is substantially as long as the cell is wide. The cell lies within an intensifying optical cavity which may be formed by a 100 percent reflecting mirror and a partially reflecting mirror both perpendicular to the line of focus of the pumping radiation. The stimulated emission from the dye material is characterized by a short pulse width and a little loss of energy between the two lasers.
One of the most highly developed fluid lasers is the AVLIS dye laser AVLIS (Atomic Vapor Laser Isotope Separation). Development of this family of lasers took place at Lawrence Livermore National Laboratory over the period of 1972 to 1999. It can produce single aperture powers approaching 3 kW with nearly diffraction limited beams. However, these lasers do not store optical energy and must be excited by an even higher peak power pump laser, albeit with much lower beam quality requirements. Rare earth ion based lasers have the advantage of long florescent lifetimes, so they require lower peak pump powers.
Liquid hosts containing rare earth have been considered. However, such devices were flash lamp driven leading to unacceptably large temperature gradients in the fluid and their poor beam quality. U.S. Pat. No. 3,779,939 to Erhard J. Schmitschek et al., assigned The United States of America as represented by the Secretary of the Navy, patented Dec. 18, 1973, shows a liquid lasing composition consisting essentially of neodymium (III) phosphorus dichloridate, retained in solution with phosphorus oxychloride by the addition of a Lewis acid.